The present invention concerns the field of integrated circuit manufacture and pertains particularly to performing a large-tilted-angle nitrogen implant into areas of a dielectric region overlaying the source/drain regions of a transistor.
As channel lengths for MOSFETs are scaled to significantly less than a micron (e.g., 0.5 microns and below), hot carrier related reliability continues to be a critical device design constraint. Reduction of the power supply voltage to 3.3 volts and below improves hot carrier lifetimes, but is not sufficient to meet reliability goals.
Incorporating nitrogen into the silicon and silicon dioxide (Si--SiO.sub.2) interfacial regions under the gate layer has been found to improve hot carrier immunity. See, for example, H. S. Momose, T. Morimoto, Y. Ozawa, M. Tsuchiaki, M. Ono, K. Yamabe and H. Iwai, Very Lightly Nitrided Oxide Gate MOSFETs for Deep-Submicron CMOS Devices, IEDM Tech. Dig., p. 359, 1991; S. Kusunoki, M. Inuishi, T. Yamaguchi, K. Tsukamoto and Y. Akasaka, Hot-Carrier-Resistant Structure by Re-Oxidized Nitrided Oxide Sidewall for Highly Reliable and High Performance LDD MOSFETs, IEDM Tech. Digest, p. 649, 1991; Y. Okada, P. J. Tobin, P. Rushbrook, and W. L. Dehart, The Performance and Reliability of 0.4 micron MOSFET's with Gate Oxynitrides Grown by Rapid Thermal Processing Using Mixtures of N.sub.2 O and O.sub.2, Trans. Electron Devices, Vol. 41, p. 191, February 1994; T. Kuroi, T. Yamaguchi, M. Shirahata, Y. Okumura, Y. Kawasaki, M. Inuishi and N. Tsubouchi, Novel NICE (Nitrogen Implantation into CMOS Gate Electrode and Source-Drain) Structure for High Reliability and High Performance 0.25 .mu.m Dual Gate CMOS, IEDM Tech Dig. p. 325, 1993.
Various techniques are available to convert the thermally grown gate oxide into oxynitride. For example, the thermally grown gate oxide may be converted into oxynitride by high temperature anneal of the grown gate oxide in NH.sub.3, N.sub.2 O or NO. Alternately, nitrogen may be implanted into the polysilicon gate. Both techniques have disadvantages.
For example, when a high temperature anneal to produce the oxynitride gate dielectric, this requires the additional high temperature processing which is incompatible with submicron device fabrication. The implantation of nitrogen and drive-in through the polysilicon gate results in fixed charge generation which degrades mobility at low electric field.